1. Field of the Invention
The present invention relates to a charged particle beam exposure apparatus using a variable rectangularly-shaped beam or a blanking aperture array and a method of cleaning such a charged particle beam exposure apparatus.
2. Background of the Invention
Recently, integrated circuit devices have widely been applied to various fields, such as computers, communications and mechanical controls with improvements in the integration density and functions. For example, DRAM devices have been improved to have increasing integration densities, such as 1 Mbits, 4 Mbits, 16 Mbits, 64 Mbits, 256 Mbits and 1 Gbits. Such an improvement in the integration density much depends on an advance of fine production techniques.
With such an improvement in the integration density, an exposure apparatus using a charged particle beam such as an electron beam have been developed in order to form fine patterns. Such a charged particle beam exposure apparatus enables a fine production equal to or less than 0.05 .mu.m with a positional precision of 0.02 .mu.m or less.
In the charged particle beam exposure apparatus, the electron beam is stabilized in order to expose a target member such as a wafer to precisely draw a pattern thereon in a predetermined position. Particularly, it is important for the electron beam not to be degraded due to a time deterioration.
In the conventional charged particle beam exposure apparatus, a deterioration in the exposure precision is caused by beam drifts. There are the following four different types of beam drifts:
(1) A charge-up drift due to a contamination on electrostatic deflection electrodes in a projection lens or a lower portion of a column tube in which the electron beam runs;
(2) A charge-up drift occurring on the upstream side of a reduction lens;
(3) A thermal drift due to heat generated in a coil of an electromagnetic deflector; and
(4) A positional drift resulting from contraction of an exposed member or a stage caused by heat generated in a focusing lens (a relative beam position drift).
Recently, it has become possible to much suppress the beam drifts (3) and (4) so that a time deterioration caused by these beam drifts is negligible by employing a thermal compensation, a thermal decomposition and/or cooling of the lens coil. However, in the present situation the charge-up drifts (1) and (2) are suppressed by disassembling the exposure apparatus, cleaning the parts thereof and replacing old parts by new ones (an overhaul).
For example, a contamination on the lower portion of the column tube causes the above charge-up drift (1). Such a contamination is caused so that a gas generated from an organic material by projecting high-energy particles onto a resist formed on the target member for exposure is deposited on the lower portion, or a carbon component contained in a gas generated by reflected electrons is evaporated on the lower portion. Hence, the contamination on the lower portion of the column tube includes insulating members. When charges such as reflected beams and secondary electrons are stored in the contamination, an electric field with respect to the potentials of peripheral portions is generated, so that the position of electron beam is varied (beam drift). Further, the charges are stored in the contamination in different ways when different patterns are exposed. Hence, it is substantially impossible to systematically evaluate the charge-up drifts and systematically perform a compensation process for compensating the charge-up drifts. In practice, it is very difficult to ensure surface cleanness of parts formed by plating or metallizing the surfaces of alumina parts. For example, a liquid for plating remains in the plated surface or a pin hole occurs therein during the cleaning process.
With the above in mind, in practice, the field correction coefficients (gain, rotation, trapezoid and offset) of the deflectors indicating the characteristics of the column tube are periodically measured. Then, the drift is separated into components, which are compensated for during the exposure process. When the quantity of drift per unit time exceeds a threshold level, the column tube is disassembled and cleaned and old parts are replaced by new ones.
The aforementioned charge-up drift (2) is caused by a contaminates deposited on slits. In order to cope with the charge-up drift (2), the field correction coefficients are periodically calculated, and the column tube is disassembled and cleaned and old parts are replaced by new ones as necessary.
Particularly, there is a problem in a charged particle beam exposure apparatus using a blanking aperture array (BAA: hereinafter such an apparatus is referred to as a BAA electron beam exposure apparatus). The final beam shape is defined by controlling the electron beams which have passed through a large number of apertures. Hence, the electron beams are directly projected onto the BAA without any break during the exposure process. As a result, organic members contained in the resist and/or the vacuum are deposited on the BAA, and the contamination is charged up and electrified very early. The charged-up BAA bends the orbits of the electron beams passing through the BAA. Hence, the electron beams may be cut by an aperture member located on the downstream side of the BAA. Further, the electron beams which should be cut by the BAA may pass through the BAA. These phenomena prevent desired patterns from being drawn on the target member. In addition, the above phenomena change with time.
In order to cope with the charge-up drift (2), overhaul of the column tube is carried out as in the case of the charge-up drift (1). For example, the BAA which has expired is replaced by a new one.
As described above, the overhaul process is needed to cope with the charge-up drifts (1) and (2), and much time and labor are needed. Further, it is necessary to adjust the apparatus after assembling the parts into the apparatus.
A method has been designed to eliminate the above-mentioned disadvantages of the electron beam exposure apparatus (see Japanese Laid-Open Patent Application No. 61-20321). The proposed method cleans the inside of the exposure apparatus in the following manner. A rod-shaped electrode is inserted into the column tube along the axis thereof. A high-frequency voltage is applied across the inner wall of the column tube and the inserted electrode, and thereby plasma is generated inside the column tube. The plasma thus generated functions to eliminate depositions on the inside of the column tube (ashing).
Japanese Laid-Open Patent Application No. 61-59826 discloses another cleaning method. Parts and electrodes located in the vacuum area in the column tube of the exposure apparatus are isolated from the ground and a high-frequency voltage is applied to the parts and electrodes. An oxygen gas has been introduced into the column tube. Oxygen gas plasma is generated by the application of the high-frequency voltage, and eliminates a contamination containing carbon compounds deposited on the parts and electrodes (ashing).
Japanese Laid-Open Patent Application No. 3-19314 discloses the following cleaning method, which will be described with reference to FIG. 1. The proposed method uses a column tube 1, which is located above a target member to be exposed. A main body 2 of the column tube 1 is made of an insulating ceramic member. T-shaped recesses 3 are formed on the inner surface of the column tube 1. This inner surface of the column tube 1 having the T-shaped recesses 3 is plated with an electrically conductive member such as gold. Thereafter, gold plated portions of parts 3a of the recesses 3 are removed by an electrical discharge machining. Electrically conductive portions 4a through 4h, formed by gold plating, are electrically isolated from each other along the optical axis of the column tube 1, so that electrodes are formed. A high-frequency signal generated by an oscillator 5 is applied across each of pairs of opposite electrodes. For the sake of simplicity, FIG. 1 shows that the high-frequency signal is applied across only a single pair of opposite electrodes.
A cleaning gas such as an oxygen gas, which is introduced into the column tube 1, becomes radical due to the application of the high-frequency signal, and cleans the inner surface of the main body 2 of the column tube 1.
However, the cleaning methods described above have the following respective disadvantages.
The cleaning method disclosed in Japanese Laid-Open Patent Application No. 61-203121 needs a complex structure which inserts the rod-shaped electrode into the column tube. An experiment conducted by the inventors shows that plasma containing a gas in the radical state cannot be widely generated stably for a long time and can be generated within a very narrow range around the rod-shaped electrode for only a very short time.
Japanese Laid-Open Patent Application N. 61-59826 discloses only the principle of the cleaning method. The inventors have also confirmed through their experiment that plasma containing a gas in the radical state cannot be widely generated stably for a long time.
Further, the inventors have confirmed through their experiment that plasma containing a gas in the radical state cannot be widely generated stably for a long time. For example, the inventors have confirmed that plasma cannot be generated under a slightly different condition and plasma is generated or not generated under the same condition.
From the above considerations, it is concluded that the above-mentioned proposed cleaning methods cannot be applied to practical use, and some improvements therein will be needed. Further, the proposed cleaning methods are not capable of efficiently cleaning, without overhaul, parts other than the electrodes, such as the BAA.